The present disclosure relates, in various embodiments thereof, to charge transport layer compositions and photoconductive imaging members comprising such compositions. In particular, the present disclosure relates to charge transport layers comprising a binder, a charge transport material, and a doping agent. The doping agent is a polyarylate material.
In the art of electrophotography, an electrophotographic imaging member or plate comprising a photoconductive insulating layer on a conductive layer is imaged by first uniformly electrostatically charging the surface of the photoconductive insulating layer. The plate is then exposed to a pattern of activating electromagnetic radiation, for example light, which selectively dissipates the charge in the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic toner particles, for example from a developer composition, on the surface of the photoconductive insulating layer. The resulting visible toner image can be transferred to a suitable receiving member such as paper. This imaging process may be repeated many times with reusable photosensitive members.
Electrophotographic imaging members are usually multilayered photoreceptors that comprise a substrate support, an electrically conductive layer, an optional hole blocking layer, an optional adhesive layer, a charge generating layer, a charge transport layer, and optional protective or overcoating layer(s). The imaging members can take several forms, including flexible belts, rigid drums, etc. For most multilayered flexible photoreceptor belts, an anti-curl layer is usually employed on the back side of the substrate support, opposite to the side carrying the electrically active layers, to achieve the desired photoreceptor flatness.
One type of multi-layered photoreceptor that has been employed as a belt in electrophotographic imaging systems comprises a substrate, a conductive layer, a charge blocking layer, a charge generating layer, and a charge transport layer. The charge transport layer often comprises an activating charge transport molecule dispersed or dissolved in a polymeric film forming binder. Generally, the polymeric film forming binder in the transport layer is electrically inactive by itself and becomes electrically active when it contains the activating molecule. The expression “electrically active” means that the material is capable of supporting the injection of photogenerated charge carriers from the material in the charge generating layer and is capable of allowing the transport of these charge carriers through the electrically active layer in order to discharge a surface charge on the active layer. The multi-layered type of photoreceptor may also comprise additional layers such as an anti-curl backing layer, required when layers possess different coefficient of thermal expansion values, an adhesive layer, and an overcoating layer. Commercial high quality photoreceptors have been produced which utilize an anti-curl coating.
U.S. Pat. No. 4,265,990 discloses a layered photoreceptor having a separate charge generating (photogenerating) layer (CGL) and charge transport layer (CTL). The charge generating layer is capable of photogenerating holes and injecting the photogenerated holes into the charge transport layer. The photogenerating layer utilized in multilayered photoreceptors includes, for example, inorganic photoconductive particles or organic photoconductive particles dispersed in a film forming polymeric binder. Inorganic or organic photoconductive materials may be formed as a continuous, homogeneous photogenerating layer.
Examples of photosensitive members having at least two electrically operative layers including a charge generating layer and diamine containing transport layer are disclosed in U.S. Pat. Nos. 4,265,990, 4,233,384, 4,306,008, 4,299,897 and 4,439,507. The disclosures of these patents are incorporated herein in their entirety.
As more advanced, complex, highly sophisticated, electrophotographic copiers, duplicators and printers are developed, greater demands are placed on the photoreceptor to meet stringent requirements for the production of high quality images. Along these lines, many photoreceptor systems have stringent requirements on the electrical properties of the photoreceptor, such as, for example, the background potential (VBG) and residual potential (Vr).
Variations in the electrical properties of a photoconductive element result in unacceptable variance in residual potential (Vr), and background potential (VBG). VBG is defined as the potential in the background or light struck areas of a photoconductive element after exposure to a pattern of activating electromagnetic radiation such as light. Unpredictable variations in VBG can adversely affect copy quality, especially in complex, high volume, high speed copiers, duplicators and printers which by their very nature require photoconductive element properties to meet precise narrow operating windows. Consequently, photoconductive elements that have poor VBG characteristics are also unacceptable or require expensive and sophisticated control systems or trained repair persons to alter machine operating parameters. Inadequate compensation of VBG variations can cause copies to appear too light or too dark. In addition, such variations in VBG properties preclude optimization of VBG properties.
Vr is defined as the remaining surface potential after full discharge from white light exposures in excess of 200 ergs-cm2. Vr and VBG impact photoreceptor development efficiency and thereby impact image quality. Relatively low Vr and VBG are desired for optimal photoreceptor performance.
Control of Vr, and VBG of photoconductive elements is important not only initially but through the entire cycling life of the photoconductive element. During the electrophotographic process, the photoconductive element is subjected to a series of charge and illumination steps which often produce changes in the electric and optical properties of the photoconductive element. These changes are called fatigue. Fatigue causes the operating characteristics to vary during the life of the photoconductive elements and is undesirable in actual commercial usage.
A common factor which produces variable Vr and VBG in photoconductive elements is the small, uncontrollable variation in acidic or basic chemical impurities in the system. Additives to the photoconductive element's layer or layers may reduce or eliminate the effects of impurities. For example, U.S. Pat. No. 4,874,682 describes a monomeric or polymeric nonvolatile basic amine incorporated in a charge transport layer to eliminate the fatigue effect of acids. In another example, U.S. Pat. No. 4,725,518, the entire disclosure of which is incorporated by reference herein, discloses addition of an aromatic amine compound and a protonic acid or Lewis acid in a charge transport layer to control Vr, dark development potential (VDDP) and VBG.
Another known treatment of photoconductive elements to control acidic or basic variations affecting Vr, VDDP and VBG involves doping the photoconductive element with other acids and bases. For example, a variance in Vr, VDDP and VBG may be controlled by the addition of trifluoroacetic acid to the transport layer in amounts ranging from about 0.1 to 100 ppm. However, the actual amount varies and must be determined by frequent measurement during the manufacturing process of the electrical behavior of the device. The dopant content is readjusted to compensate for the quantity of acid necessary to achieve the desired electrical specifications. This acid doping procedure is tedious, time-consuming and difficult to predictably control. Additionally, while the use of acids to dope the charge transport layer is known to lower Vr and/or VBG, these lower potentials are reduced at the expense of dark decay.
U.S. Pat. No. 6,337,166 discloses a wear resistant charge transport layer comprising at least a polycarbonate polymer binder having a number average molecular weight of not less than 35,000, at least one charge transport material, polytetrafluoroethylene particle aggregates having an average size of less than about 1.5 microns and a fluorine-containing polymeric surfactant dispersed in a solvent mixture of at least tetrahydrofuran and toluene. The dispersion forms a uniform stable material. U.S. Pat. No. 6,326,111 discloses adding hydrophobic silica to such a composition.
U.S. Pat. No. 5,164,276 describes photoreceptors that incorporate a dopant in one or both of the charge generation layer or the charge transport layer. The dopant includes organic molecules containing basic electron donor or proton acceptor groups. Preferred dopants include aliphatic and aromatic amines, triethanolamine, n-dodecylamine, n-hexadecylamine, tetramethyl guanidine, 3-aminopropyltriethoxy silane, 3-aminopropyltrihydroxysilane and its oligomers.
U.S. Pat. No. 5,356,741 describes a process for controlling variations in electrical characteristics of a electrophotographic imaging device by eliminating the effect of acidic and basic impurities in a photoconductive element. The process includes coating a substrate with a first dispersion to form a charge generating layer, and then coating with a second dispersion to form a charge transport layer, wherein at least one of the first or second dispersions includes a solution of weak acid or weak base and the conjugate salt of weak acid or weak base in an amount effective to reduce variations in dark development potential and background potential characteristics of an imaging device.
It is still desirable to provide a charge transport layer composition that offers improved photoreceptor performance. Along these lines, it is desirable to provide a charge transport layer composition that allows for increased photoreceptor sensitivity via improved electrical properties. It is desirable to provide a charge transport layer composition for a photoreceptor that reduces at least one of the Vr and VBG. It is also desirable to provide a charge transport layer composition for a photoreceptor that reduces at least one of the Vr and VBG and lowers the dark decay of the photoreceptor.